1. Field of the Invention
The present invention relates to a cell reselection method for a mobile subscriber, and more specifically, to a cell reselection method for a mobile subscriber using a decision based on C1 and C2 parameters.
2. Description of the Prior Art
With the rapid development of wireless technology, wireless communication and personal mobile communication has become an integral part of daily life. Global System for Mobile Communication (GSM) is one of the most commonly used communication systems in the world. The widespread use of GSM cellular phones allows for an easier and more convenient way of communication. Within the communication range, people can talk to others anytime they want regardless of their current locations.
Please refer to FIG. 1. FIG. 1 is a diagram of a wireless communication system 6. Wireless communication system 6 comprises a plurality of cells (not shown in FIG. 1) within which a base station provides communication services. These cells are called cellular wireless communication systems because of their structural resemblance to honeycomb. For the ease of explanation, FIG. 1 comprises a cell 22 and a cell 30. A base station 24 provides communication services within cell 22 and a base station 28 provides communication services within cell 30. A mobile subscriber (MS) 26, located within both the service ranges of cell 22 and cell 30, is used to transmit and receive wireless communication signals and to communicate with the base stations. Since the MS 26 is located within the service range of both cell 22 and cell 30, the MS 26 can receive and send signals from/to base station 24 and base station 28 simultaneously. Usually the MS 26 chooses the base station capable of providing stronger signals as its serving cell, camps on the serving cell and starts to execute subsequent jobs. When the movement of the MS 26 causes a certain degree of change in the signal strength of the serving cell, the MS 26 will execute a confirming step of the current serving cell it is camping on. This confirming step is called cell selection using two parameters C1 and C2 for judgment wherein C1 is the path loss criterion and C2 is the cell reselection criterion. In the prior art, a serving cell having a positive C1 value and the highest C2 value corresponding to an MS is considered the best serving cell. So once an MS detects a base station of a higher C2 value than that of the current serving cell, it begins to perform cell reselection.
Please refer to FIG. 2. FIG. 2 is a flowchart illustrating a prior art cell reselection method of an MS. As previously described, a base station of the strongest signal strength will be selected as the serving cell of an MS. The MS calculates the C1 and C2 parameters of a base station based on the signals sent by the base station and these parameters are then used for the cell reselection process. According to European Telecommunications Standards Institute (ETSI) 05.08, the C1 parameter is defined as follows:C1=(A−Max(B,0));where A=RLA_C−RXLEV_ACCESS_MIN;    B=MS_TXPWR_MAX_CCH−P; and    RLA_C is the average power of the signal carrier wave;    RXLEV_ACCESS_MIN is related to the minimum signal that the operator wants the network to receive when being initially accessed by an MS;    MS_TXPWR_MAX_CCH is the maximum power that an MS may use when initially accessing the network;P is the maximum output power of the MS.
If the C1 parameter of a base station is greater than zero, it means this base station is suitable for an MS to camp on, and the base station of the greatest C1 will be selected as the serving cell. If the MS is moving, indicating that the C1 parameter it detects also changes accordingly, or if a selected base station refuses to provide communication services for other reasons, the C2 parameter has to be taken into account for cell reselection. According to ETSI 05.08, the C2 parameter is defined as follows:C2=C1+CELL_RESELECTION_OFFSET−TEMPORARY OFFSET*H(PENALTY_TIME−T);where for non-serving cellsH(x)=0 for x<0=1 for x≧0; andfor serving cellsH(x)=0.
CELL_RESELECTION_OFFSET is a value given to a cell to encourage or discourage an MS to reselect that cell. PENALTY_TIME is the time period during which a negative offset is given to C2 of a cell after an MS has put it on the list of the strongest cells. TEMPORARY_OFFSET is the amount of the negative offset given during the above-mentioned PENALTY_TIME.
After an MS has camped on a chosen serving cell, the MS periodically detects the signals of a plurality of neighboring cells and updates the C1 and C2 parameters. In FIG. 2 step 100 executes this procedure of updating C1 and C2. Step 110 judges if the neighboring cell and the serving cell belong to the same local area (LA). If in step 110 the judgment is “yes”, then step 120 judges if the C2N parameter of the neighboring cell remains larger than the C2S parameter of the serving cell for a predetermined period. If in step 110 the judgment is “no”, then step 130 judges if the C2N parameter of the neighboring cell remains larger than the sum of the C2S parameter of the serving cell and a first threshold broadcasted hysteresis value (CRH) for a predetermined period. In other words, step 130 judges if the equation C2N>C2S+CRH holds for a predetermined period. If the judgment in step 120 or step 130 is “yes”, step 140 then executes cell reselection. If the judgment in step 120 or step 130 is “no”, then cell reselection is not executed and step 100 is repeated for updating C1 and C2. In FIG. 2 a process 700 including step 110, step 120 and step 130 is called better-cell decision.
Please refer to FIG. 3. FIG. 3 is a diagram of a wireless communication system 50. In FIG. 3 The MS 26 is located within the service ranges of base station cell A, base station B and base station C at the same time. The user of the MS 26 is an employee working in an office, and the office is located in the intersection of cell ranges provided by base station A, base station B and base station C. If the MS 26 camps on base station A in the first place and the user moves slightly, the MS 26 might have to execute cell reselection and camp on to base station B. If the user frequently moves around within a small region in their office in a day, the MS 26 will perform several needless cell reselections and consume a lot of power. In fact, the MS 26 does not need cell reselection since within a small region, by camping on a fixed base station the MS 26 can receive good communication service without wasting power. However, the cell reselection method in the prior art cannot solve this problem.